As power using equipment which uses a portable power source system, there are for example electric tools such as electric screwdrivers for use in the screw fastening work. The use of electric tools can significantly improve work efficiency. While electric tools have recently been available at home improvement centers and the like and come to be used for do-it-yourself, they have been mainly used in construction sites and the like for business purpose.
There are however some cases where a construction site or the like is not wired. Further, the presence of cords or the like may negatively affect working performance. For this reason, a battery pack is employed in electric tools which operate at relatively low voltage, such as screwdrivers. A typical battery pack accommodates plural secondary batteries therein as a power source. Such a battery pack is detachable and, for conducting charging, it is inserted into a charger specific to the battery pack. On a construction site or the like, even when a battery is run down halfway through the work, preparation of a spare battery pack allows replacement of the battery pack and continuation of the work.
The battery pack (Model No. EZ9025) for the electric tool (DRILL & DRIVER, Model No. EZ6225) manufactured by Matsushita Electric Works, Ltd., one of the examples of the conventional battery packs, is described by reference to FIGS. 15 to 17.
FIG. 15 is an oblique view of the battery pack (EZ9025), and FIG. 16 is a top plan view thereof. At the time of insertion of the battery pack into the electric tool, the battery pack is positioned by a guide 1 to be inserted, and then fixed to the electric tool with a latch 2. On the upper end of the battery pack disposed while being exposed to the outside are a positive electrode terminal 3 both for charging and discharging, a charge negative electrode terminal 4 specifically for charging, a discharge negative electrode terminal 5 specifically for discharging, and a thermistor terminal 6.
FIG. 17 shows a circuit diagram of the battery pack (EZ9025). A discharge circuit comprising the positive electrode terminal 3, a secondary battery 7 and the discharge negative electrode terminal 5 is not provided with a current regulating device, whereas a charge circuit comprising the positive electrode terminal 3, the secondary battery 7 and the charge negative electrode 4 is provided with a control circuit 8 and a thermal protector 9, securing safety during charging. Further, a thermistor 10 is connected to the thermistor terminal 6. During charging, the temperature of the battery pack is monitored with resistance values of the thermistor 10 to control charging according to the monitored temperatures.
As thus described, the conventional battery pack comprises the current regulating device in the charge circuit. The current regulating device will prevent a current from continuing to flow in the battery pack if the charger should suffer a breakdown. However, the discharge circuit is not provided with a current regulating device because there are cases where a heavy current temporarily flows and a total amount of the discharge current is regulated by the capacity of the secondary battery.
In the conventional battery pack, it is possible to charge the battery pack by connection between the discharge terminal and a large-sized battery such as a battery for automobiles, since the discharge terminal is in a state of exposure to the outside. There however is a safety problem in charging by the use of the discharge terminal because of the absence of the current regulating device in the discharge circuit.
For the purpose of protecting terminals of a battery pack from shock caused by dropping and the like, there has been proposed a structure where an external terminal is provided on the inner circumferential side of a lid of a loop-shaped battery pack (Japanese Laid-Open Patent Publication No. 2001-135287). Also in this structure, however, the discharge terminal is in a state of exposure to the outside and the connection between the discharge terminal and the large-sized battery is therefore possible.